Adaptive-rate cardiac pacemakers in which the stimulation frequency or rate is set as a function of signals picked up in the body of the patient that reflect the physiological requirements of the patient with regard to cardiac activity have been known and been in clinical use for years in many versions. A survey of the goals pursued in the development of rate adaptation in pacemaker technology and the relevant paths taken is given in K. Stangl et al, Frequenzadaptive Herzschrittmacher [Adaptive-Frequency Cardiac Pacemakers], Darmstadt, 1990.
Many arrangements for measuring impedance in the area of the chest or the heart to obtain an impedance signal for adaptive-rate cardiac pacemakers are also known, and thus the technology of intracardial impedance measurement per se is familiar to one skilled in the art. Most of these arrangements are designed to obtain a signal that represents the tidal volume or the cardiac output as an expression of the patient's physical exertion level and as an actual rate control parameter; for this aspect, see for instance European Patent Disclosures EP 0 151 689 B1 and EP 0 249 818, or German Patent Disclosure DE 42 31 601 A1.
The so-called ResQ method (for Regional Effective Slope Quality) is also known (Max Schaldach, Electrotherapy of the Heart, First Edition, Springer-Verlag, pp. 114 ff.), in which the course over time of intracardial impedance is used to determine the physiologically appropriate adaptive heart rate.
This method is based on the recognition that the intracardial impedance has an especially significant dependency on the exertion level of the organism within a certain time slot after a QRS complex--the so-called "region of interest" or ROI.
The slope of the impedance curve in the ROI is therefore determined, and the difference between the slope of a resting or reference curve and the slope of the currently measured impedance curve (exertion curve) is calculated. The adaptive heart rate is set as a function of this difference. The association of the calculated slope difference with the heart rate to be set is done here as well by means of a characteristic curve. Since this curve differs for different people and is dependent on the physical condition, the cardiac pacemaker must be calibrated individually for each patient, and the calibration must be repeated if the patient's health and capacity for exertion changes, or if his life circumstances change, and in that case then the location of the ROI must be checked as well.
Various suggestions for self-adaptation (autocalibration) of adaptive-rate cardiac pacemakers are found in European Patent Disclosure EP 0 325 851 A2, U.S. Pat. No. 5,074,302, or European Patent Disclosure EP 0 654 285 A2. U.S. Pat. No. 5,303,702 shows a trend calculation for the self-adaptation of a pacemaker controlled on the basis of the cardiac output.
International Patent Disclosure WO 93/20889 shows a two-sensor arrangement with one circuit for detecting the cardiac output and with an additional activity sensor, in which the stimulation rate is determined as a function of the target rates that can be derived for the individual sensors.
European Patent Application EP 97 250 057.3 shows an impedance-controlled pacemaker is proposed, which is capable of making do without a patient-specific calibration and adapts automatically to altered peripheral conditions. In this pacemaker, the time integral of the impedance over a predetermined portion of the heart cycle is determined as a primary impedance variable and used for rate adaptation.